This invention relates to the problem of accurately determining the location of the liquid surface in a well drilled into the subsurface of the earth. More particularly, this invention relates to method and apparatus for economically obtaining the necessary information from a well which will enable one to accurately and efficiently determine the location of the liquid surface therein. This accurate determination of the liquid surface enhances greatly the ability of one skilled in the art to analyze well problems and provide curative measures therefore to increase a given well's production capability.
While the prior art has provided various devices and methods to determine the location of the liquid surface in a given well all have failed to provide one skilled in the art with an apparatus and technique which substantially eliminates the effects of human error, weather conditions and changing downhole conditions on such determination. Representatives of the previous devices include the Sonolog instrument made by the Keystone Development Corporation of Houston, Tex., the Echometer instrument made by the Echometer Company of Wichita Falls, Tex., and the device described in Mobil Oil Corporation's U.S. Pat. No. 4,318,278. Both the Echometer and the Sonolog instrument suffer from the dependence on a field operator to make frequent and multiple calculations and interpretations which provide the opportunity for gross error. These instruments require an operator to trigger a pressure pulse source, record as a function of time the amplitude of pulse reflections in the wellbore annulus on a constant speed chart recorder. These recorded reflections are analyzed by the operator by counting tubing collars and attempting to choose the location on the chart of the liquid reflection. These devices clearly do not provide for frequent economic, efficient and accurate measurement of the liquid surface over any significant period of time and allow for a great deal of human error.
The device and method described in the aforementioned Mobil patent was an improvement over the Echometer and Sonolog devices but also suffers from many deficiencies which are remedied by the invention of the present application.
First of all, the prior device utilizes a gating means which is responsive to the output of a transient pressure transducer for starting and stopping a counting means. When the gating means detects an initiating pressure pulse, it starts the counting means which begins accumulating clock pulses, one for each foot of depth. The counting means accumulates clock pulses until stopped by the gating means. The gating means stops the counting means when an output from the transient pressure transducer is sufficiently large. The number of pulses accumulated in the counting means represents the depth of the liquid surface. Since the gating means is responsive to the amplitude of the output of the transient pressure transducer, any out-put which is sufficiently large will cause the gating means to stop the counting means. There is no test provided to determine if the output from the transient pressure transducer is the reflection from the liquid surface. As the pressure increases in the well annulus, the acoustic transmission properties improve and all the reflections in the wellbore increase in amplitude. The reflection from a tubing anchor, a large collar or any anomaly may become large enough to prematurely activate the gating means and stop the counting means and cause the accumulated clock pulses to represent a depth other than that of the liquid surface.
A second deficiency in the prior device concerns the use of a manually adjustable pulse counter for producing one pulse output for an adjustable number of clock pulse inputs. This manual adjustment is employed to account for the pulse velocity in the well annulus. The value established by the field operator is one of trial and error interpretation. At the beginning of the test, the rate of the pulse counter is established by the field operator, such that the time between succeeding pulses is equal to the time required for sound to travel one foot in the annulus and return to the source. Even if the operator does the calibration perfectly, the device will often give erroneous depth information shortly after the well is shut-in due to a change in the pulse velocity in the well annulus caused by i.e., change in pressure, change in gas constituency or a temperature gradient through the annulus.
A third deficiency in the prior device concerns the use of a calibrated mute time for gating out the initiating pressure pulse and reflections, thereby rendering the gating means inoperative for a known, adjustable period of time beginning with the initiating pressure pulse. If the liquid level in the well annulus rises to a point that the liquid surface reflection time becomes less than the mute time, the device may be disabled for protracted periods of time.
A fourth deficiency in the prior device concerns the use of an adjustable trigger level to activate the gating means for stopping the counting means. Once the gating means has started the counting means, the counting means will accumulate clock pulses until stopped by the gating means. Since the gating means is responsive to any output from the transient pressure transducer which is greater in amplitude than the adjusted trigger level, when an output from the transient pressure transducer is greater in amplitude than the trigger level the gating means will respond by stopping the counting means. In the event a field operator erroneously sets the trigger level, the gating means will fail to respond to the output of the transient pressure transducer and result in loss of data.
A fifth deficiency in the prior device concerns the use of digital readout therein which remains at the well site. The use of digital readout and its attendant support circuitry and the installation thereof in the device merely adds unnecessarily to the overall cost of same and its use, i.e., power cost and the reduction of the time on the well between battery charges. Further, large heavy-duty batteries are required to operate the device for a sufficient period of time to complete a well test. Safety problems present a concern when loading, unloading, connecting and/or charging such batteries as are required to power the device.
A sixth deficiency in the prior device concerns the utilization of a single pressure transducer therein. The appropriate pressure transducer is determined by the field operator based on (1) maximum surface pressure he expects the well to reach during the test, and (2) what pressure transducers he has available to him at the time. Thus, he must estimate such maximum surface pressure. The transducer may be damaged if the surface pressure during the test exceeds the maximum pressure rating of the transducer the operator chooses to install. If, from his available stock, the field operator chooses an inappropriate transducer the pressure reading will clearly be less than accurate. Since the surface pressure during a test can range from low to high, in order to provide the necessary resolution, different transducers should be utilized as the pressure changes. This is practically impossible with only one transducer in the device
A seventh deficiency in the prior device concerns the recording of pressure as a digital count which is displayed and/or printed as a digital count. This digital count must be converted to pressure by computation on the part of the field operator creating a situation for human error.
An eighth deficiency in the prior device concerns the necessity of an experienced, skilled field operator to correctly operate the device. The field operator is required to use judgment and experience to set up the device correctly and operate switches in the correct sequence or a large amount of data is lost.
A ninth deficiency in the prior device concerns the fact that the measurement of depth is affected by the energy in the initiating pressure pulse. The reflection of the pressure pulse from the liquid surface does not arrive at the transient pressure transducer as an instantaneous pulse. Dispersion of the pressure pulse in the well annulus and low pass filtering of the transient pressure transducer output result in a signal which arises from 0 to its maximum value in fifty to one hundred milliseconds. The use of a set trigger level in conjunction with the relatively slow rise time can result in reflection arrival time variations of almost 100 milliseconds which represents between 30 to 70 feet. The operator normally attempts to set the adjustable trigger level and acoustic amplifier/gain such that the reflected pressure pulse from the liquid surface is twice the voltage of the trigger level. When a well is shut-in the acoustic characteristics therein change, i.e., normally the gas becomes more conductive to acoustical energy. Thus, the acoustic energy reflected by the liquid surface and received by the transient pressure transducer becomes greater and as the transient pressure transducer output increases in amplitude, the time required for the signal to reach the trigger level decreases making the liquid surface appear shallower than it actually is. Conversely, in the event the solenoid-operated valve which creates the pressure pulse is slow or sluggish (due to temperature or battery age) the resulting pulse can have relatively low energy. In the event the reflected pressure pulse from the liquid surface is just large enough to trigger the gating means, the arrival time will be about 50 milliseconds later than it would be if the reflected energy were twice as large resulting in the liquid surface appearing deeper than it actually is.
A tenth deficiency of the prior device concerns the necessity of having a field operator at the site at all times to adjust the device for changing conditions in the well during a test. For example, in the absence of the field operator, if the (1) surface pressure increases sufficiently some pressure data will be lost and the pressure transducer can be damaged, (2) if the surface pressure decreases sufficiently the pressure resolution will become very poor, and (3) if acoustic properties change sufficiently all data from that point forward will be lost.